Aluminum alloy strips for heat exchangers

ABSTRACT

Aluminum alloy strips less than 0.3 mm thick for making heat exchangers, containing, in wt. %: Si&lt;1.0, Fe&lt;1.0, Cu&lt;0.8, Mg&lt;1.0, Mn≦1.8, Zn&lt;2.0, In&lt;0.2, Sn&lt;0.2, Bi&lt;0.2, Ti&lt;0.2, Cr&lt;0.25, Zr&lt;0.25, Si+Fe+Mn+Mg&gt;0.8, other elements &lt;0.05, each and &lt;0.15 in total. The strips have between the surface and half the thickness a difference of corrosion potential, measured relative to a saturated calomel electrode in accordance with the ASTM G69 standard, of at least 10 mV. The invention also concerns a method for making such strips by continuous casting in conditions promoting formation of segregations in the strip core, optionally hot rolling, cold rolling optionally with one or several intermediate or final annealing(s) of 1 to 20 hours at a temperature between 200 and 450° C. The fins or separators made from the inventive strips have enhanced resistance to perforating corrosion.

DOMAIN OF THE INVENTION

The invention relates to the domain of thin strips (thickness<0.3 mm)made of aluminium alloy for making heat exchangers, particularly heatexchangers used for cooling engines and for air conditioning passengercompartments in motor vehicles. The aluminium alloy strips forexchangers are used either bare or coated on one or both faces with abrazing alloy. The invention more particularly relates to uncoatedstrips used for fins or separators fixed on tubes or elements in contactwith the cooling fluid.

STATE OF THE ART

Aluminium alloys are now very widely used in the manufacturing of heatexchangers for automobiles due to their low density, which enables aweight saving particularly compared with copper alloys, while providinggood heat conduction, ease of installation and good corrosionresistance. These exchangers comprise tubes for circulation of internalheating or cooling fluid and fins or separators for heat transferbetween the internal fluid and the external fluid, and they aremanufactured either by mechanical assembly or by brazing.

In addition to their heat transfer function, the fins or separators mustprovide protection for tubes against perforation due to the galvaniceffect, in other words making the fins from an alloy that has anelectrochemical corrosion potential lower than for the tubes, such thatthe fin acts as a sacrificial anode. The alloy most frequently used fortubes at the moment is the 3003 alloy, therefore an alloy of the sametype is usually used for the fins, with an addition of 0.5 to 2% ofzinc. The composition of the 3003 alloy registered at the AluminumAssociation is as follows (% by weight):

Si<0.6; Fe<0.7; Cu=0.05-0.2; Mn=1.0-1.5; Zn <0.1.

Strips made of this type of alloy are usually obtained by semicontinuouscasting of a plate, homogenisation of this plate, hot rolling, then coldrolling possibly with intermediate annealing and/or final annealing.They can also be obtained by continuous casting of strips between twobelts (twin-belt casting) or between two cooled rolls (twin-rollcasting). It is known that when the twin-roll casting technique is usedto obtain Al—Mn alloys with a fine grain structure, the blank ishomogenised to eliminate segregations derived from casting, which givesa good compromise between mechanical strength and formability. Theseproperties are described in particular in patent EP 0039211 (AlcanInternational) for alloys containing between 1.3 and 2.3% of manganese,and in U.S. Pat. No. 4,737,198 (Aluminum Company of America) for alloyscontaining from 0.5 to 1.2% of iron, less than 0.5% of silicon and 0.7to 1.3% of manganese that can be used for the manufacture of exchangerfins. Patent application WO 98/52707 issued by the applicant describes aprocess for manufacturing aluminium alloy strips containing at least oneof the elements Fe (from 0.15 to 1.5%) or Mn (from 0.35 to 1.9%) withFe+Mn<2.5% and possibly containing Si (<0.8%), Mn (<0.2%), Cu (<0.2%),Cr (<0.2%) or Zn (<0.2%) by continuous casting between cooled rolls androlled to a thickness between 1 and 5 mm followed by cold rolling, theforce applied to the casting rolls expressed in tonnes per metre widthof strip being less than 300+2000/e, where e is the strip thicknessexpressed in mm. The use of these strips for making brazed exchangerfins is mentioned.

Patent application WO 00/05426 by Alcan International describes themanufacture of strips for aluminium alloy fins with composition:Fe=1.2-1.8%, Si=0.7-0.95%; Mn=0.3-0.5%, Zn=0.3-2% by continuous castingof strips at a cooling rate or more than 10° C./s.

Patent applications WO 01/53552 and WO 01/53553 by Alcan Internationalalso concern the manufacture of strips for iron alloy fins containing upto 2.4% of iron by continuous casting and very fast cooling. The purposeis to obtain a more negative corrosion potential.

OBJECTIVE OF THE INVENTION

Although fins or separators have to provide a galvanic protection fortubes, they must not be excessively damaged by corrosion during the lifeof the heat exchanger. Sufficient integrity of the material has to bemaintained, because if it is perforated too quickly, the heat exchangewill not be as efficient due to the loss of useful area. The fin couldeven be separated from the tube, which would prevent heat conductionbetween these components. Thus, the purpose of the invention is toobtain strips for heat exchanger fins or separators made of aluminiumalloy that will be used particularly in the automobile industry, thathave good mechanical strength, good formability and good resistance toperforating corrosion while acting as a sacrificial anode.

PURPOSE OF THE INVENTION

The purpose of the invention is aluminium alloy strips withthickness<0.3 mm, to be used in the manufacture of heat exchangers, withcomposition (% by weight): Si<1.5; Fe<2.5; Cu<0.8; Mg<1.0; Mn=<1.8;Zn<2.0; In<0.2; Sn<0.2; Bi<0.2; Ti<0.2; Cr<0.25; Zr<0.25;Si+Fe+Mn+Mg>0.8, other elements <0.05 each and <0.15 in total, theremainder being aluminium, with a difference of corrosion potentialbetween the surface and the mid-thickness, measured with respect to asaturated calomel electrode according to ASTM standard G69 equal to atleast 10 mV.

The invention also relates to a method for manufacturing such strips bycontinuous casting under conditions that promote the formation ofsegregations in the strip core, possibly hot rolling, cold rollingpossibly with one or more intermediate or final annealing(s) lasting for1 to 20 h at a temperature of between 200 and 450° C.

DESCRIPTION OF THE FIGURES

FIG. 1 shows the variation of the corrosion potential, measured withrespect to a saturated calomel electrode, of a strip according to theinvention made from the alloy in example 1 as a function of the depthfrom the surface.

Similarly, FIG. 2 shows the variation of the corrosion potential of astrip made from the alloy according to example 2.

DESCRIPTION OF THE INVENTION

The applicant found that by using continuous casting for type 3000(Al—Mn) or type 8000 (Al—Fe) alloys, possibly with added zinc, underparticular casting conditions and with an appropriate transformationprocedure, strips could be obtained with a corrosion potential gradientthrough their thickness and that this property encourages lateralpropagation of corrosion rather than propagation perpendicular to thesurface, which achieves the sacrificial effect while avoidingperforation and therefore damage to the fin or the separator with time.This potential gradient is at least 10 mV. According to one assumptionmade by the inventors, this difference could be related to the presenceof segregations in the strip core under particular selected castingconditions, a phenomenon that would normally be avoided and that leadsto differences in the composition in solid solution within the thicknessof the strips.

The zinc content varies as a function of the alloy used for the tubes,so as to obtain an electrochemical potential difference between thetubes and the fins that is both sufficient for the fin to act assacrificial anode, and is not too high so that it does not deterioratetoo quickly. The corrosion potential of the fin or separator can belowered by also adding indium, tin and/or bismuth up to a content of0.2%. For 3003 alloy tubes, the zinc content is preferably between 1.0and 1.5%. For tubes made of an Al—Mn alloy with a higher content ofzinc, for example such as alloys with more than 0.4% copper described inpatent application EP 1075935 issued by the applicant, the zinc contentshould be kept below 0.8%.

The copper content is preferably kept below 0.5%. The possible additionof up to 0.2% of titanium, up to 0.25% of zirconium and/or up to 0.25%of chromium improves the resistance of the alloy when hot (SAGresistance).

In a first variant of the invention, the alloy used is a 3003 type alloywith a zinc content of up to 2%, in other words an alloy with thefollowing composition (% by weight):

Si<1.0; Fe<1.0; Cu<0.8; Mg<1.0; Mn=0.8-1.8 Zn<2.0; In<0.2; Sn<0.2;Bi<0.2; Ti<0.2; Cr<0.25; Zr<0.25; other elements <0.05 each and <0.15 intotal, the remainder being aluminium.

The addition of silicon, preferably more than 0.5% and up to 1%,contributes to increasing the solidification interval of the alloy,which encourages the occurrence of segregations during casting. At morethan 1%, there is a risk of reaching the alloy burning temperatureduring the exchanger brazing operation.

In a second variant of the invention, an alloy in the 8000 series isused with the following composition (% by weight):

Si=0.2-1.5; Fe=0.2-2.5; Cu<0.8; Mg<1.0; Mn=<1.0; Zn<2.0; In<0.2; Sn<0.2;Bi<0.2; Ti<0.2; Cr<0.25; Zr<0.25; Si+Fe>0.8, other elements <0.05 eachand <0.15 in total, the remainder being aluminium.

One particularly suitable composition range is as follows:

Si=0.8-1.5; Fe=0.7-1.3; Mn<0.1; Cu<0.1; Mg<0.1, and preferablySi=1.0-1.3 and Fe=0.9-1.2.

The method for manufacturing strips according to the invention includesthe production of the alloy from a filler adjusted to obtain therequired alloy composition. The metal is then cast continuously in theform of a strip with a thickness of between 1 and 30 mm, either by atwin-belt casting to between 12 and 30 mm, or preferably by castingbetween two cooled rolls shrinked with shells to a thickness of between1 and 12 mm. Unlike what is stated in patent application WO 98/52707,the casting parameters are chosen to encourage the appearance ofrelatively important segregations in the cast strip core.

In the case of twin-roll casting, the contact between the metal and thecooled cylinders should be as good as possible, so as to increase thetemperature gradient at the metal surface during casting, whichencourages segregations. The different parameters on which action can betaken are particularly the length of the contact arc between the metaland the rolls, the force applied by the rolls during casting and thetemperature of the roll shells. A long contact arc, preferably longerthan 60 mm, encourages the formation of segregations. Similarly, a highforce, preferably more than 100+2000/e t/m of width of the cast strip,where e is the thickness of the cast strip expressed in mm, alsoencourages segregation. Finally, the temperature of the shells must beas low as possible, and preferably less than 100° C.

The cast strip may be hot rolled and then cold rolled in the case oftwin-belt casting. However, twin-roll cast strip is cold rolleddirectly. If the final thickness is fairly small, then at least oneintermediate annealing is necessary at a temperature of between 200 and450° C. If the metal has to be delivered in the annealed temper, thenannealing at a temperature of between 200 and 450° C. is carried out onthe rolled strip until the final thickness. If the metal is delivered inthe strain-hardened temper, the transformation procedure is adapted suchthat the reduction ratio is adjusted to the target strain-hardeningratio.

Strips according to the invention are used to make heat exchanger finsor separators with high mechanical strength, so that the thickness canbe reduced below the thicknesses used for fins or separators accordingto the prior art, while maintaining good formability. In service, thefin or the separator acts as a sacrificial anode, but corrosionprogresses laterally parallel to the surface, which avoids or retardsperforation, assures integrity of the tube-fin assembly and therefore acontinuous heat exchange. Strips with a coarse grain microstructure arefavourable to hot resistance during brazing.

EXAMPLE Example 1

An alloy with the following composition (% by weight) was prepared in amelting furnace: Si Fe Cu Mn Mg Cr Ni Zn Ti 0.80 0.55 0.10 1.0 0.0690.002 0.005 1.4 0.015

A 5 mm thick strip was cast on a Jumbo 3 Cm™ continuous castinginstallation made by the Pechiney Rhenalu Company with a width of 1420mm with a force between rolls equal to 780 t, a 70 mm contact arc wasobtained with the temperature of the roll shells being equal to 70° C.The strip was then cold rolled in a single pass down to a thickness of0.7 mm and was then subjected to 12 h intermediate annealing in an airfurnace programmed to 520° C. to bring the metal to a temperature of theorder of 380° C., and cold rolled in three passes down to 130 μm.

A first part of the strip was subjected to a recovery annealing for 2 hat 350° C., followed by rolling to 100 μM. A second part was subjectedto recrystallisation annealing for 2 h at 400° C., and then rolled to100 μm. Finally, the same annealing was carried out in a third part,which was rolled to 75 μm. For comparison, 3003 zinc alloy strips weremade with the following composition: Si Fe Cu Mn Mg Cr Ni Zn Ti 0.220.57 0.12 1.15 — — — 1.4 —using the same manufacturing procedure, but starting from a verticalsemicontinuous casting process with a recovery annealing for 2 h at 350°C., and rolling up to 100 μm.

Static mechanical properties of these strips were measured including theyield shength R_(0.2), tensile strength R_(m) and elongation A. Theresults are shown in table 1: Thickness R_(0.2) R_(m) A Procedure (μm)(MPa) (MPa) (%) CC annealed at 100 235 248 3.2 350° C. CC annealed at100 188 197 2.4 400° C. CC annealed at 75 213 227 1.8 400° C. CVannealed at 100 158 162 1.5 350° C.*CC = continuous casting; CV = vertical semicontinuous casting.

It is found that the metal obtained by continuous casting has both abetter mechanical strength and better elongation than metal derived fromtraditional casting.

The variation of the corrosion potential through the thickness comparedwith a saturated calomel electrode according to ASTM standard G69 wasmeasured on the 75 μm strip. The figure shows the presence of a zoneunder the surface and at a depth of about 15 μm, in which the potentialquickly changes from −890 mV to −870 mV.

Example 2

An alloy was prepared with the following composition (% by weight): SiFe Cu Mn Mg 1.2 1.1 <0.1 <0.1 <0.1

A 6.1 mm thick and 1740 mm wide strip was cast on a Davy™ continuouscasting installation made by the Pechiney Eurofoil Company, with a forcebetween the rolls of 550 t, a contact arc of 60 mm and a temperature ofthe roll shells equal to 42° C. The strip was then cold rolled to athickness of 80 μm to obtain an H19 type metallurgical temper.

The mechanical characteristics of this strip are as follows: R_(m) (MPa)R_(0.2) (MPa) A % 311 256 7.3

It can be seen that this metal produced by continuous casting has anexcellent compromise between mechanical strength and elongation.

The metal was then subjected to a typical brazing cycle in a furnaceunder a nitrogen atmosphere, comprising a 2-minute plateau at 600° C.

The mechanical characteristics obtained after this treatment are asfollows: R_(m) (MPa) R_(0.2) (MPa) A % 135 53 13.2

The yield strength after brazing, R_(0.2), equal to 53 MPa issignificantly better than the value obtained for the 3003 alloy stripsused traditionally, obtained by conventional casting (of the order of40-45 MPa).

From the point of view of corrosion resistance, a variation of thecorrosion potential occurs within the thickness of the metal with these8xxx alloys, as can be seen in FIG. 2, always in relation to the castingprocess used; the beneficial nature of this variation has been explainedabove for 3xxx alloys.

It would be possible to add zinc to adapt the corrosion potential to thecorrosion potential of alloys used for tubes with which the separatorswill be coupled, since zinc has only a very minor influence on themechanical characteristics or the thermal conductivity.

1. Aluminum alloy strips with thickness<0.3 mm, to be used in themanufacture of brazed heat exchangers, with composition consistingessentiall of (% by weight): Si<1.5; Fe<2.5; Cu<0.8; Mg<1.0; Mn≦1.8;Zn<2.0; In<0.2; Sn<0.2; Bi<0.2; Ti<0.2; Cr<0.25; Zr<0.25;Si+Fe+Mn+Mg>0.8, other elements <0.05 each and <0.15 in total, with adifference of corrosion potential between the surface and themid-thickness, measured with respect to a saturated calomel electrodeaccording to ASTM standard G69 equal to at least 10 mV.
 2. Stripsaccording to claim 1, characterised in that the zinc content is between1.0 and 1.5%.
 3. Strips according to claim 1, characterised in that thezinc content is below 0.8%.
 4. Strips according to claim 1,characterised in that the copper content is below 0.5%.
 5. Stripsaccording to claim 1, characterised in that they are made from an alloywith the following composition: Si<1.0; Fe<1.0; Cu<0.8; Mg<1.0;Mn=0.8-1.8 Zn<2.0; In<0.2; Sn<0.2; Bi<0.2; Ti<0.2; Cr<0.25; Zr<0.25;other elements <0.05 each and <0.15 in total, the remainder beingaluminum.
 6. Strips according to claim 5, characterised in that thesilicon content is between 0.5% and 1%.
 7. Strips according to claim 1,characterised in that they are made from an alloy with the followingcomposition: Si=0.2-1.5; Fe=0.2-2.5; Cu<0.8; Mg<1.0; Mn≦1.0; Zn<2.0;In<0.2; Sn<0.2; Bi<0.2; Ti<0.2; Cr<0.25; Zr<0.25; Si+Fe>0.8, otherelements <0.05 each and <0.15 in total, the remainder being aluminum. 8.Strips according to claim 7, characterised in that they are made from analloy with the following composition: Si=0.8-1.5; Fe=0.7-1.3; Mn<0.1;Cu<0.1; Mg <0.1.
 9. Strips according to claim 8, characterised in thatthe silicon content of the alloy is between 1 and 1.3%.
 10. Stripsaccording to claim 8, characterised in that the iron content is between0.9 and 1.2%.
 11. Method for manufacturing strips according to claim 1,by continuous casting at a thickness of between 1 and 30 mm underconditions that promote the formation of segregations in the strip core,possibly hot rolling, cold rolling possibly with one or moreintermediate or final annealing(s) lasting for 1 to 20 h at atemperature of between 200 and 450° C.
 12. Method according to claim 11,characterised in that continuous casting is made between two cooled andshrinked rolls.
 13. Method according to claim 12, characterised in thatthe force applied by the rolls during casting is more than 100+2000/et/m of width of the cast strip, where e is the thickness of the caststrip expressed in mm.
 14. Method according to claim 12, characterisedin that the contact arc between the metal and the rolls is longer than60 mm.
 15. Method according to claim 12, characterised in that thetemperature of the shells is less than 100° C.